Metal transfer devices known as “launders” are widely used for transferring liquid metal in metal refining and processing plants, for example from a furnace to a mould. A typical launder comprises a trough made of a refractory material, through which the metal flows under the influence of gravity.
Launders may be either unheated or heated. Heated launders are preferred for certain applications, as they help to maintain the temperature of the metal as it is transferred. Preheating the launder also reduces the thermal shock on the refractory material as the liquid metal is introduced, thereby reducing the risk of cracking.
An example of a heated launder is described in US 2010/0109210 A1. This device includes a trough body for carrying liquid metal, a heating element positioned adjacent the trough body, an insulating layer and an outer shell defined by a bottom and two side walls. The trough body is made of a thermally conductive castable refractory material, which allows heat to be transferred from the heating elements to the liquid metal. The thermal conductivity of the trough body depends on the refractory material from which it is made, being in the range of about 9 to 11 W/m·K for silicon-carbide based refractories, but only about 1.5 to about 1.9 W/m·K for alumina-based refractories. As a result, the efficiency of heat transfer is limited, particularly when an alumina-based refractory is used.
Another problem is that if the trough body cracks, it may be possible for liquid metal to leak through to the heating elements, which could be damaged by contact with the liquid metal.
WO2012/0175911A describes another heated launder, which includes a cast trough body for receiving liquid metal, a metallic shell and a filler layer comprising a cast refractory material between the trough body and the shell. The filler layer has a high thermal conductivity to transfer heat efficiently from a heater to the trough body. The filler layer and the metallic shell prevent any leakage of liquid metal if the cast trough body cracks.
We have found that in certain circumstances differential thermal expansion of the filler layer and the metallic shell can cause the shell to become distorted. Also, if the liquid metal is held in the launder for an extended period of time, it may be difficult to achieve the heat input necessary to maintain the metal in a liquid state.
It is an object of the present invention to provide a metal transfer device that mitigates one or more of the aforesaid disadvantages.